1. Field of the Invention
The invention relates to a capable method of improving crystal quality of a chemical compound semiconductor device such as a visible semiconductor laser diode.
2. Description of the Related Art
A conventional method for forming crystals includes a vapor phase epitaxy process (MO-VPE) utilizing thermal organic metal decomposition. FIG. 1 illustrates crystal layers grown by the MO-VPE process. As illustrated, on an n-type GaAs substrate 1 are successively grown a n-type GaAs buffer layer 3, a n-type (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P clad layer 4 (0&lt;x.ltoreq.1), a Ga.sub.0.5 In.sub.0.5 P active layer 5, a p-type (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P clad layer 6 (0&lt;x.ltoreq.1), a p-type Ga.sub.0.5 In.sub.0.5 P hetero buffer layer 8, a n-type GaAs block layer 10, and a p-type GaAs contact layer 12. For instance, see Japanese Unexamined Patent Public Disclosure No. 3-53577.
However, the above mentioned conventional method has a problem. In the conventional method, when the n-type GaAs buffer layer 3, the n-type GaAs block layer 10 and the p-type GaAs contact layer 12 are to be grown, triethyl gallium (TEG) is used as a source of gallium (Ga). The use of TEG causes the generation of an intermediate product by a memory effect in the crystal growth equipment such as trimethyl indium (TMI)and triethyl indium (TEI) both of which are used as a source of indium (In) for (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P crystal growth (0.ltoreq.X.ltoreq.1). Thus, the indium (In) is incorporated into the GaAs layers, and they form InGaAs layers.
As illustrated in FIG. 2A, the thus formed InGaAs layers have a lattice constant J rather than a lattice constant I of a GaAs substrate, and produce a lattice mismatch between the GaAs substrate and (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P crystals (0.ltoreq.X.ltoreq.1) which have been formed so that they have the same lattice constant as that of the GaAs substrate.
If an electrical current is continuously applied to a semiconductor laser composed of such crystals having a lattice mismatch, there would be an increase in &lt;110&gt; dark line defects. As a result, as illustrated in FIG. 2B, the operating current is increased with the result that the semiconductor laser is degraded and finally no longer oscillates.